Leaving aside the issue of better ways to parse a CSV file, and single letter var names. But fixing use strict/warnings via my, and fixing bareword filehandles and 2 arg opens, and fixing that changing %hash1 did nothing to what was in @a

#!/usr/bin/perl
use strict;
use warnings;

open my $fh2,"<","file2.txt";
my %hash2; 

while (my $line=<$fh2>){ 
  chomp $line; 
  my @k = split ',', $line;
  $hash2{$k[0]} = $k[1];
  } 
close $fh2; 

open my $fh1,"<","file1.txt";
my @final; 
while (my $line=<$fh1>){ 
  chomp $line; 
  my @k = split ',', $line;
  $k[1] = $hash2{$k[0]} if (exists $hash2{$k[0]});
  push @final,join(',',@k); 
  } 
close $fh1; 

open my $fh3,">","file1.txt";
for my $value (@final) {print $fh3 $value."\n";}
close $fh3;
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